1. Field of the Invention
The present invention relates generally to RMS-to-DC converters, and more particularly, to an RMS-to-DC converter that utilizes balanced squaring cells and is capable of measuring true power at microwave frequencies.
2. Description of the Related Art
RMS-to-DC converters are used to convert the RMS (root-mean-square) value of an arbitrary signal into a quasi-DC signal that represents the true power level of the signal. Numerous techniques have been devised for performing RMS-to-DC conversions. One of the most fundamental is known as the "thermal" method. With the thermal method, the signal is used to generate heat in a resistive dissipator. The heat is then measured, usually by establishing a temperature balance using a second dissipator. The DC input to the second dissipator then provides a measure of the RMS value of the signal. Another technique involves "computing" converters which utilize nonlinear analog signal processing. Examples of computing converters include an "explicit" converter, which utilizes an amplitude squaring cell followed by a filter and then a square rooter, and an "implicit" converter which utilizes an absolute value cell followed by a squarer-divider and a filter embedded in the a feedback loop.
Another type of computing converter which operates on the "difference of squares" principle. This circuit utilizes a differential input, four-quadrant multiplier and shares some of the features of both the thermal technique, and the previously described computing techniques. Like the advanced thermal techniques, it seeks to null the difference between the square of the input and the DC output. However, like the other computing converters, it utilizes nonlinear signal processing elements. All of these techniques are discussed more thoroughly in an article by Barrie Gilbert: "Novel Technique For R.M.S.-D.C. Conversion Based On The Difference Of Squares," Electronics Letters, Apr. 17, 1975, Vol. 11, No. 8, pp. 181-182.
Although the techniques discussed above can provide an accurate measure of the true RMS value of a signal at relatively low frequencies, they do not operate well at microwave frequencies, i.e., upwards of 1 GHz. Signal measuring devices capable of operation at microwave frequencies are available, e.g., diode detectors, but they are not true RMS detectors. Instead, they are essentially "envelope" detectors which respond to the amplitude of the modulation envelope of a signal (and power indirectly), rather than responding inherently to the power of a complex waveform such as a CDMA carrier and its noise-like modulation.
Accordingly, a need remains for an improved technique for measuring the true RMS value of a signal.